Islamabad: The Pakistani army deployed troops on the streets of the capital on Sunday after thousands of stone-throwing supporters of an Islamist assassin who was hanged last month clashed with police in chaotic scenes.

Mumtaz Qadri was executed on February 29 for killing a Punjab governor over his call for blasphemy reform in what analysts said was a "key moment" in Pakistan's long battle against religious extremism.

But it has also exposed deep religious divisions in the conservative Muslim country of 200 million.

An estimated 25,000 supporters of the former police bodyguard gathered in Islamabad's twin city Rawalpindi in the afternoon to offer prayers, before turning toward the heavily-barricaded capital, which was patrolled by hundreds of police and paramilitary soldiers.Riot police carrying batons and shields fired tear gas to try to prevent them pushing closer to the city centre. Some protesters set fire to a container while chanting slogans.

A military spokesman tweeted late Sunday that the army had been requisitioned to "control" the situation and secure the Red Zone around Parliament, where protesters were gathering.

The protest had been largely ignored by the Pakistani media, which has increasingly become subject to government-ordered news blackouts designed to prevent unrest from spiralling out of control.

Qadri was working as a bodyguard for Punjab governor Salmaan Taseer when he shot him in 2011 over the governor's call to reform the blasphemy law, which critics say is frequently misused to oppress religious minorities.

Sunday's unrest came hours after a group of angry men assaulted a former pop star accused of insulting the Prophet Mohammed's youngest wife, in the latest case of vigilante violence linked to blasphemy.

Ex-singer Junaid Jamshed, who is now a prominent Sunni evangelist, was leaving Islamabad airport on Saturday night when he was set upon by a group of around six men who were waiting to attack him at the exit.

The incident was captured on mobile phone video and has been widely viewed.

Jamshed was forced to flee back into the airport. In a later Facebook posting, he said it was time for the nation to decide it "will not let these religious fanatics prevail amongst us".

Most Internet users condemned the violence on social media.

"Get hold of the culprits and make an example of them. Enough of this madness in the name of love," wrote Facebook user Karami Elahi.

Many pointed to the fact that police and security seemingly failed to intervene, despite the outbreak of violence at one of the country's busiest airports.

A spokesman for the Airport Security Force was not available for comment despite repeated attempts.

A blasphemy case was brought against Jamshed by the Sunni Tehreek religious group, which organised Sunday's protests in Islamabad, in December 2014.